If you mean a pi alternative as in a single board computer with an ARM processor then probably the RockPro64. This is what powers my Tanto Ronin Dojo node but it should work fine, if not better than a pi, for a LN node.

Not as cheap as a rpi but I think a better choice would something like an Intel NUC.

Used Dell Optiplex are cheap and powerful if you have cheap energy

Pierre Rochard made Node Launcher for Lightning Node on Desktop. But unfortunately the project han't been updated in a long time. I hope it get's picked up again one day because I thought it was really cool.

Spare laptops are probably your best bet. Built in battery backup and you can use Unraid or Portainer for web docker management.

ASUS mini PC. It has a variety of CPUs and allows SSD and HDD modules. Low power.

https://www.asus.com/Displays-Desktops/Mini-PCs/PN-PB-series/Mini-PC-PN50/

Not sure whether this would work but I'd be very interested to see if it could be done. I think the continual writing on to the SD, and db size will become a problem.

Even if the storage and other other specs seemed similar to a Pi, the difficulty to maintain regular upgrades, installing and uninstalling various tools.

If the question is for running a node in a fairly serious attempt to either route payments not just for your own payments, you'd need to maintain a synced blockchain, and keep synced to the lightning graph as much as possible.

I'd go with one of the other small form-factor cpus, nuc, laptop or desktop. Laptops work well for me.

If you're looking for more UMPF I'd say HX90 Ryzen 9 / 64gb RAM. pref. with dual M2 SSD in Raid1 setup.

Check out the TinyMiniMicro playlist by ServeTheHomme

I just ordered an HP Elitedesk 800 G3 Mini ~$130 on ebay.

Odroid & raspibolt (not documented, but mostly obvious howto)